##
The Effective Medium Approximations: Some Recent Developments

#### David Stroud, Department of Physics,
The Ohio State University, Columbus, Ohio 43210-1106

We provide a brief and highly selective review of the Landauer-Bruggeman
effective-medium approximation as applied to random composite media.
We first discuss this approximation as applied to linear composites, i.
e., to those materials in which there is a linear relation between
a curl-free electric field and a divergence-free current density. We then
describe extensions of this approach to random composites with cubic
nonlinearities in addition to a dominant linear term, and to composites in
which the components have a fluctuating conductivity (``conductivity
noise''). Finally, we mention one novel application: to conductivity noise
in a random composite of normal metal (N) and perfect conductor (S). It
is shown that the extension of the EMA leads to a prediction of a frequency
range in which the conductivity noise has a 1/omega frequency
dependence, even if the noise in the individual components is
frequency-independent.
PACS Nos. 42.70.Nq, 42.65.Pc, 78.66.Sq